Threshold trigger having regenerative feedback but maintaining conduction only during presence of input signal



Apnl 2, 1963 A. CLELAND 3,084,265

THRESHOLD TRIGGER HAVING REGENERATIVE FEEDBACK BUT I MAINTAININGCONDUCTION ONLY DURING PRESENCE OF INPUT SIGNAL Filed June a, 1959 AllLl Alllllllll l i S """v' "V" "I Y K R Q g INVENTOR Aaron Zyle CZelazzdl1 W m, LAW MM v ATTORN 3,084,265 THRESHGLD TREGGER HAViNG REGENERATEVEFEEBBACK BUT MAENTAENHNG CUNBUCTHQN GNLY DURENG PRESENCE F INPUT Si-GNALAm'on Lyle Cieland, Manvilie, N..i., assignor to Aircraft RadioCorporation, Boonton, Ni, a corporation oi New Jersey Filed June 8,1959, Ser. No. 818,594 8 (Jlairns. (Cl. EFL-S35) This invention relatesto a threshold trigger circuit of the type which is rendered operativeupon receipt of a signal and continues in operation as long as thesignal persists. More specifically, the invention concerns a thresholdtrigger circuit utilizing transistors which is rendered operative uponreceipt of a signal to actuate a switch or other load and continues inoperative condition until the signal terminates at which time thetrigger circuit becomes inoperative.

A major object of this invention is to provide a threshold triggercircuit which draws very little current from a power supply while in itsready condition awaiting receipt of a signal to become operative.

Another object of the invention is to provide a simple threshold triggercircuit which is light in weight and particularly adapted for use inaircraft to respond to the receipt of a radio signal for operating anindicator lamp during the period which the radio signal persists.

In the attainment of these objects an important feature of the inventionresides in the arrangement of a pair of transistors of oppositeconductivity types which are normally biased to cut oil but directlyconnected together so when one transistor is rendered conductive itapplies potentials to the other transistor opposing the biasingpotentials; and including a regenerative feedback circuit eliective whenthe other transistor is conducting for applying potentials to the onetransistor in opposition to biasing potentials. Signals supplied to thebase of the first transistor cause the trigger circuit to becomeconductive so the output of either transistor may be utilized to operatea switch such as a switching transistor.

Where an alternating current signal is applied to the base or the firsttransistor, a capacitor is connected to the coupling circuit andelectrodes of the first transistor so it becomes charged as the firsttransistor conducts during alternate half-cycles of the alternatingsignal and thereby applies a potential to the second transistorsufiicient to render that transistor conductive whereupon theregenerative feedback circuit becomes efiected. It is essential that theregenerative feedback circuit apply potentials to the first transistorwhich oppose the normal biasing potentials and thereby render the firsttransistor more conductive, but which are insufficient to continueconduction of the first transistor in the absence of a signal applied toits base.

These and other objects, advantages and features of the invention willbecome more apparent from the following specification taken inconjunction with the accompanying drawing which illustrates a thresholdtrigger circuit embodying the invention in its preferred form.

As shown in the drawing the threshold trigger circuit includes a firsttransistor 11 connected in circuit with a potential source so it isnormally biased to cut off. Toward that end, this transistor 11, whichis of the NPN type has its emitter 12 connected through resistor 13 toground and its collector 14 connected through a load resistor 15 to asource 16 of positive potential which in this case may be about 10volts. Base 17 of the transistor 11 is connected to an intermediatepoint 18 of a voltage divider formed by resistors 19 and 21 extendingbetween potential source 22, of about 10 volts, and

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ground. Resistors l9 and 21 are so selected that base 17 is biasedslightly above ground but insufficient to render the transistor 11conductive when emitter 12 is near ground potential and collector 14 hasa positive potential of about 10 volts.

From the output of transistor 11 at collector l4 and across loadresistor 15 a coupling lead 23 is connected to base 24 of a secondtransistor 25, of the PNP type, which is likewise normally biased to cutoh by having its emitter 26 connected through emitter follower resistor27 to a source 28 of positive potential of about 10 volts and itscollector 29 connected through load resistor 31 to ground. If transistor11 is rendered conductive, as by adjusting the potential of its base 17,the potential of base 24 is lowered with respect to emitter 26sufiiciently to render transistor conductive. When this occurs, thevoltage across emitter follower resistor 27 is applied through couplingresistor 33 in a regenerative feedback circuit to emitter 12 oftransistor 11. In this way, the potential of emitter 12 is lowered torender the transistor 11 more conductive. However, the regenerativefeedback potential applied to emitter 12 is insufficient to rendertransistor 11 conductive in the absence of a signal of a threasholdlevel applied to base 17 in addition to the biasing potential from thevoltage divider of resistors 19 and 21. Emitter 12 is normally biasedslightly above ground by its connector between resistors 13 and 33 whichare connected in series with resistor 27 between source 28 and ground.

Should a DC. potential of the desired threshold level be applied topoint 18 and base 17, transistor 11 would become conductive upon theincrease of the potential of base 17. When transistor 11 thereby beginsto conduct, the voltage at collector 14 is reduced by reason of thecurrent drawn through load resistor 15 so the coupling lead 23 lowersthe potential of base 24 thereby rendering transistor 25 conductive.This causes a flow of current in emitter follower resistor 27 whichlowers the potential of emitter 26 and this lowered potential issupplied through the regenerative feedback circuit to the emitter 12 ofthe transistor 11 thereby rendering the transistor 11 more conductive.This, in turn, further reduces the potential on base 23 so continuedregenerative action increases conduction of transistor 25 to itssaturation point. In this way, as long as the signal ap plied at point18 and base 17 persists, both transistors continue to conduct and theoutput which appears across load resistor 31 in the collector circuit oftransistor 25 may be utilized to operate a switching transistor 35 whichin turn may control a signal light 36 if desired. As soon as the signalapplied at point 18 is removed or falls below the threshold level,transistor 11 returns to its non-conductive state and the potential ofbase 24 is raised to cut off transistor 25 so the signal light 35 willbe extinguished.

Since in the usual case an alternating current signal is available in aradio receiver for purposes of indicating the presence of such a signal,the drawing shows such a signal source at 36 coupled through a condenser37 to the point 13 and the base 17. When an alternating current signalis thus applied to base 17, transistor ll. acts as a rectifier and isrendered conductive on alternate haltcycles of the signal from source 36which raise the potential of base 17. This conduction of transistor 11on alternate half-cycles develops a direct current potential acrosscapacitor 38 connected between emitter 12 and collector M. A charge oncapacitor 33 lowers the potential of collector l4 and coupling lead 23to lower the potential of base 24. This charge on the capacitor 33 isalso effective through resistor 33 to raise the potential on emitter 26.Changes in potentials applied to the electrodes of transistor 25 as aresult of charging condenser osages 38; render transistor 25 conductiveto cause current to flow through resistor 27 and, accordingly, a drop inpotential at the emitter connection of resistor 27. This potential atemitter 26 is fed back to emitter 12 of transistor 11 to cause thistransistor to conduct and thus initiate regenerative action to rendertransistor 2'5 heavily conductive. Through this regenerative feedbacktransistor 25 will remain conductive as long as an adequate signal isapplied to base 17 of transistor 11. When transistor 11 is fullyconductive, capacitor 38 is discharged through emitter 12 and collector14 and the regenerative feedback connection will maintain conduction ofboth transistors until the signal applied to the base 17 of transistor11 falls below the threshold level.

By way of example, the following is a tabulation of values which may beused for the components of the circuit although it will be apparent thatmany variations are possible without departing from the invention:

Inasmuch as during normal standby operation tran sistors' 11 and 25 areboth non-conductive, the current drain on the system supplyin thecircuitis at a minimum, Whenever a signal is applied to base 17 of thethreshold level, both transistors become conductive and continue in astate of conduction as long as the signal on base 17 persists atthreshold level. Immediately following the termination of the signal atbase 17 both transistors return to their non-conductive state. It isunnecessary that the current in the two transistors be equal and in factwhatever load is placed on transistor 25 has no material effect on theoperation of transistor 11.

As the circuit shown in the accompanying drawing and described in theforegoing specification is but one pre ferred embodiment of theinvention, it is intended that it be construed in an illustrative senseas many modifications will become apparent without departing the scopeof the invention as defined in the appended claims.

In the claims:

1. A threshold trigger circuit comprising a'pair of transistors ofopposite conductivity types connected in circuit with a potential sourceto normally apply cut off bias potentials to electrodes of bothtransistors, coupling means from one transistor to the other transistoreffective upon conduction of said one transistor for applying potentialsopposing the bias potentials on the electrodes of said other transistor,means for applying a signal to die base of said one transistor forinitiating conduction of said other transistor, and a regenerativefeedback circuit between corresponding electrodes of said transistorsefiective upon conduction of said other transistor for applyingpotentials opposing the bias potentials on the electrodes of said onetransistor but insufficient to render said one transistor conductive inthe absence of a signal applied to the base thereof.

2. A threshold trigger circuit comprising a pair of transistors ofopposite conductivity types, each of said transistors having an inputelectrode, an output electrode and a third electrode, respectively,means for applying cut-oil biasing potentials to the electrodes of saidtransistors, means connecting the output electrode of one of saidtransistors to the input electrode of the other of said transistors,means for applying a signal to the input electrode of said onetransistor for initiating conduction of said other transistor, andregenerative feedback means between said third electrodes eitective onconduction of said other transistor for applying potentials overcomingthe cut-off bias potentials on the electrodes of said one transistor torender said one transistor conductive.

3. A threshold trigger circuit comprising a pair of transistors ofopposite conductivity types, each of said transistors having an inputelectrode, an output electrode, and a third electrode, respectively,means for applying cut-off biasing potentials to the electrodes of saidtransistors, means connecting the output electrode of one of saidtransistors to the input electrode of the other of said transistors,means for applying a signal to'the input electrodes of said onetransistor for initiating conduction of said other transistor, and aregenerative feedback circuit including a resistive element between saidthird electrodes effective on conduction of said other transistor forapplying potentials overcoming the cutoff bias potentials on theelectrodes of said one transistor to render said one transistorconductive.

4. A threshold trigger circuit comprising a pair of transistors ofopposite conductivity types connected in circuit with a potential sourceto normally apply cut off bias potentials to electrodes of bothtransistors, coupling means from the collector of said one transistor tothe base of said other transistor effective upon conduction of said onetransistor for applying a potential to the base of the other transistoropposing the cut oil bias potentials applied to electrodes of saidother'transistor, means for applying a signal to the base of said onetransistor, and a regenerative feedback circuit between the emitters ofsaid transistors effective upon conduction of said other transistor forapplying a potential to the emitter of said one transistor in oppositionto the cut oif bias potentials applied toelectrodes of said onetransistor but insufiicient to render said one transistor conductive, inthe absence of a signal applied to its base.

5. A threshold trigger circuit comprising a pair 05 transistors ofopposite conductivity types connected in circuit with a potential sourceto normally apply cut of? bias potentials to both transistors, couplingmeans from one transistor to the other transistor effective uponconduction of said one transistor for applying a potential opposing thebias potentials on the electrodes of said other transistor, means forapplying an alternating current signal to the base of said onetransistor for rendering said one transistor conductive on alternatehalf-cycles of the signal, a capacitor connected between an electrode ofsaid one transistor and said coupling means to be charged by saidconduction of said, one transistor on alternate half-cycles of saidsignal for applying the potential of the charge to said other transistorin opposition to the bias potentials applied to electrodes of said othertransistor, and a regenerative feedback circuit between electrodes ofsaid transistors etr'ective'upon conduction of said other transistor forapplying a potential opposing the biaspotentials on the electrodes ofsaid one transistor but insutficient to. render said one transistorconductive in the absence of a signal.

6. A threshold trigger circuit comprising a first transistor of the NPNtype having its electrodes connected to a potential source to normallyapply cut elf bias potentials to electrodes of said first transistor andincluding a load resistor in the collector circuit, means for applying asignal to the base of said first transistor, a second transistor of thePNP type having its electrodes connected in circuit with a potentialsource to normally apply cut off bias potentials to electrodes of saidsecond transistor and including a resistor connected between saidpotential source and the emitter of said second transistor, a couplingcircuit between the collector of said first transistor and the base ofsaid second transistor for applying a po-v tential to the base of saidsecond transistor effective upon conduction of said first transistor foropposing the cut off bias potentials applied to electrodes of saidsecond transistor, and a regenerative feedback circuit connected between the emitter of said second transistor and the emitter of saidfirst transistor elfective upon conduction 05 said second transistor forapplying a potential to the emitter of the first transistor opposing thecut otI bias potentials applied to electrodes of said first transistorbut insufiicient to render said first transistor conductive in theabsence of a signal applied to its base.

7. A threshold trigger circuit comprising a first transistor of the NPNtype having its electrodes connected in circuit with a potential sourceto normally apply cut ofi bias potentials to electrodes of said firsttransistor, means for applying an alternating current signal to the baseof said first transistor for rendering said first transistor conductiveon alternate half-cycles of said signal, a second transistor of the PNPtype connected in circuit with a potential source to normally apply cutotf bias potentials to electrodes of said second transistor, couplingmeans between the collector of said first transistor and the base ofsaid second transistor eflective upon conduction of said firsttransistor for applying a potential to the base of said secondtransistor in opposition to the cut ofi bias potentials applied toelectrodes of said second transistor, a regenerative feedback circuitbetween the emitter of said second transistor and the emitter of saidfirst transistor etfective upon conduction of said second transistor forapplying a potential to the emitter of said first transistor inopposition to the cut off bias potentials applied to electrodes of saidfirst transistor and condenser means providing a low impedancealternating current path between the collector and emitter of said firsttransistor to be charged by conduction of said first transistor duringalternate half-cycles of said signal for applying through said couplingmeans a potential to the base of said second transistor for renderingsaid second transistor conductive in opposition to bias potentialsapplied to electrodes of said second transistor.

8. A threshold trigger circuit comprising a pair of transisters 01'}opposite conductivity types, biasing means normally rendering each ofsaid transistors non-conductive including a first voltage divider havingintermediate points respectively connected to the emitter electrodes ofthe transistors and a second voltage divider having an intermediatepoint connected to the base electrode of one of said transistors, saidbiasing means establishing a threshold level for said one transistor, adirect connection between the output electrode of said one transistorand the input electrode of said other transistor, means for applying asignal to the base electrode of said one transistor, and a regenerativefeedback circuit between the emitter electrodes of said transistorsincluding a part of said first voltage divider between said intermediatepoints effective to render both said transistors conductive when thesignal applied to the base electrode of said one transistor is above thethreshold level.

References Cited in the file of this patent UNITED STATES PATENTS2,759,104 Shellett Aug. 14, 1956 2,798,160 Bruck July 2, 1957 2,840,727Guggi June 24, 1958 2,843,743 Hamilton July 15, 1958 2,843,762 TrentJuly 15, 1958 2,853,633 McVey Sept. 23, 1958 2,858,456 Royer Oct. 28,1958 2,905,815 Goodrich Sept. 22, 1959 2,971,134 Cockrell Feb. 7, 1961FOREIGN PATENTS 789,478 Great Britain Jan. 22, 1958 OTHER REFERENCESTransistor Circuit Handbook, Garner, 1957 edition, published byEducational Book Co. Publishing Division Coyne Electrical School.

1. A THRESHOLD TRIGGER CIRCUIT COMPRISING A PAIR OF TRANSISTORS OFOPPOSITE CONDUCTIVITY TYPES CONNECTED IN CIRCUIT WITH A POTENTIAL SOURCETO NORMALLY APPLY CUT OFF BIAS POTENTIALS TO ELECTRODES OF BOTHTRANSISTORS, COUPLING MEANS FROM ONE TRANSISTOR TO THE OTHER TRANSISTOREFFECTIVE UPON CONDUCTION OF SAID ONE TRANSISTOR FOR APPLYING POTENTIALSOPPOSING THE BIAS POTENTIALS ON THE ELECTRODES OF SAID OTHER TRANSISTOR,MEANS FOR APPLYING A SIGNAL TO THE BASE OF SAID ONE TRANSISTOR FORINITIATING CONDUCTION OF SAID OTHER TRANSISTOR, AND A REGENERATIVEFEEDBACK CIRCUIT BETWEEN CORRESPONDING ELECTRODES OF SAID TRANSISTORSEFFECTIVE UPON CONDUCTION OF SAID OTHER TRANSISTOR FOR APPLYINGPOTENTIALS OPPOSING THE BIAS POTENTIALS ON THE ELECTRODES OF SAID ONETRANSISTOR BUT INSUFFICIENT TO RENDER SAID ONE TRANSISTOR CONDUCTIVE INTHE ABSENCE OF A SIGNAL APPLIED TO THE BASE THEREOF.